1. Field of the Invention
A radar system typically includes an antenna that mechanically or electronically scans a target. Such a radar system can be used, for example, in search and rescue operations and as a weather or mapping radar. The resolution of the radar system in the scanning directions will be affected by the characteristics of a radiation pattern of the antenna. For example, smearing is a phenomenon that occurs when a target is made to look much bigger than it actually is because of limits in the resolution caused by the antenna beam shape.
The invention relates generally to improving the resolution of a radar system, and more particularly to compensating for the effects of smearing in the scanning direction.
2. Description of the Prior Art
As shown in FIG. 1, a radar system, for example an airborne search and rescue radar, may have an antenna 105 that scans for targets 110 in the azimuth direction. As the antenna scans across a target, e.g., a small boat, the radar system may receive multiple returns depending upon the beamwidth 115 of the antenna radiation pattern in the azimuth plane.
In the resulting radar system output, as shown in FIG. 2, the region 205 illuminated on the display 210 which corresponds to the target may be broadened, i.e., smeared, in the azimuth dimension. The smearing of the radar output reduces the azimuth resolution of the system so that multiple targets that are positioned close together in azimuth may be indistinguishable.
In a typical system, the azimuth resolution is limited approximately to the half power beamwidth (HPBW) of the antenna. For example, if the antenna has a HPBW of 10xc2x0, a small target will appear on the radar screen as an arc, concentric about the aircraft, of approximately 10xc2x0, even though the target actually may only intersect a fraction of a degree in the azimuth plane. In such a case, multiple targets that are separated by less than 10xc2x0 in azimuth may be indistinguishable, i.e., they will not appear as distinct targets.
Prior systems, such as synthetic aperture radar (SAR) have employed signal processing (typically optical or digital) to improve azimuth resolution. In a typical SAR system, an antenna is positioned on an aircraft that is moving at a constant altitude and velocity, and azimuth scanning occurs linearly, at a rate equal to the velocity of the aircraft. Such a configuration is impractical for applications such as search and rescue, which require radial scanning.
More fundamentally, SAR systems rely on the interference phenomenon of complex signals and achieve increased azimuth resolution by coherently processing received radar signals, i.e., by processing amplitude and phase information. Consequently, SAR systems generally are significantly more complex and costly than corresponding non-coherent radar systems.
There is a need for a relatively simple, economical radar system that can improve azimuth resolution.
It is a general object of the present invention to provide a radar system with improved resolution in the scanning direction.
It is another object of the present invention to provide a radar system with improved resolution in the scanning direction having a radial scanning rate compatible with search and rescue applications.
It is another object of the present invention to provide a radar system that can improve resolution in the scanning direction by processing received signals using antenna pattern information, in order to reduce cost and complexity.
In accordance with the objects described above, one aspect of he present invention provides a smearing compensation apparatus for a radar system having a scanning antenna. The apparatus includes a first memory for storing a plurality of data streams, with each data stream corresponding to a radar signal received at a different angular position of the antenna. A second memory stores radiation pattern data of the antenna. A frequency domain transform processor performs a frequency domain transform on the radiation pattern data stored in the second memory to produce a frequency characteristic of the radiation pattern data. A compensation function processor applies a compensation function to the frequency characteristic of the radiation pattern data to produce a compensated frequency characteristic, and an inverse frequency domain transform processor performs an inverse frequency domain transform on the compensated frequency characteristic to produce digital filter coefficients. A digital filter processes the plurality of data streams stored in the first memory using the digital filter coefficients.
Another aspect of the present invention provides a radar system including the above described apparatus and, in addition, a scanning antenna, a transmitter for generating radar signals and outputting the radar signals to the antenna, and a receiver for receiving radar signals from the antenna.
Aspects of the present invention may include one or more of the following features. The radiation pattern data stored in the second memory may include an azimuth pattern of the antenna, or may include a plurality of azimuth patterns of the antenna, with each azimuth pattern corresponding to an elevation angle.
Each of the plurality of data streams may include information for all target ranges of the radar system at a particular angular position of the antenna. A third memory may store a summation of a plurality of data streams corresponding to the same angular position of the antenna, and an averaging processor may compute an average of the plurality of data streams corresponding to this angular position.
The compensation function may be a multiplicative inverse, and the frequency domain transform may be a discrete Fourier transform. The inverse frequency domain transform may be an inverse discrete Fourier transform.
Another aspect of the present invention provides smearing compensation apparatus for a radar system having a scanning antenna. The apparatus includes a first memory for storing a plurality of data streams, with each data stream corresponding to a radar signal received at a different angular position of the antenna, and a second memory that stores digital filter coefficients. A digital filter processes the plurality of data streams stored in the first memory using the digital filter coefficients stored in the second memory. The digital filter coefficients are determined by performing a frequency domain transform on radiation pattern data of the antenna to produce a frequency characteristic of the radiation pattern data, applying a compensation function to the frequency characteristic of the radiation pattern data to produce a compensated frequency characteristic, and performing an inverse frequency domain transform on the compensated frequency characteristic.
Another aspect of the present invention provides a smearing compensation apparatus for a radar system having a scanning antenna. The apparatus includes a first memory for storing a plurality of data streams, with each data stream corresponding to a radar signal received at a different angular position of the antenna, and a second memory for storing radiation pattern data of the antenna. A transform means performs a frequency domain transform on the radiation pattern data stored in the second memory to produce a frequency characteristic of the radiation pattern data. A compensation means applies a compensation function to the frequency characteristic of the radiation pattern data to produce a compensated frequency characteristic. An inverse transform means performs an inverse frequency domain transform on the compensated frequency characteristic to produce digital filter coefficients. A digital filter processes the plurality of data streams stored in the first memory using the digital filter coefficients.
Another aspect of the present invention provides a smearing compensation method for a radar system having a scanning antenna. The method includes the step of storing in a first memory a plurality of data streams, each data stream corresponding to a radar signal received at a different angular position of the antenna, and storing radiation pattern data of the antenna in a second memory. A frequency domain transform is performed on the radiation pattern data stored in the second memory to produce a frequency characteristic of the radiation pattern data. A compensation function is applied to the frequency characteristic of the radiation pattern data to produce a compensated frequency characteristic. An inverse frequency domain transform is performed on the compensated frequency characteristic to produce digital filter coefficients. The plurality of data streams stored in the first memory is processed with a digital filter using the digital filter coefficients.
These and other objects, features and advantages of the present invention will become more apparent upon a consideration of the following description of the preferred embodiments of the present invention taken in conjunction with the accompanying drawings.